FIX API (financial information exchange - application programming interface) is a protocol designed specifically for traders and investors who want to connect directly to trading servers without going through trading platforms like the MT4 or MT5. With FIX API, traders can thus connect to a broker using a trading platform or interface of their choice, while still keeping latency to a minimum.
FIX API solutions are usually used by traders with very specific trading needs only. This could for example include advanced algorithmic traders or high-frequency traders. Also, keep in mind that some brokers require quite substantial minimum deposits and/or trading volumes from a client in order to make the FIX API solution available.